This invention relates to composite optical elements, optical isolators, and optical attenuators for applications in optical communications and optical measurements, and also to processes for producing the same.
Japanese Patent Application Kokai No. 11-2725 describes a composite optical element comprising a Faraday rotator and at least a first and a second birefringent regions joined to one plane of the rotator, with or without at least a third and a fourth birefringent regions joined to the opposite plane of the rotator, and also describes optical isolators, optical circulators, and optical switches utilizing such optical elements. In order to achieve a desired optical isolation or the like, it is essential that the first and second birefringent regions and, where provided, the third and fourth birefringent regions should be precisely equal in thickness. According to the cited teachings, the first and second birefringent regions are made by: first forming grooves in plate materials, equidistantly in parallel with one another to a predetermined depth by etching or other similar method, leaving ridges or lands in between; the grooved sides of the plate materials are faced to each other, with the grooves and lands of one side being fitted in the lands and grooves of the other, and joined integrally with the aid of adhesive; and then the both outer sides of the joined body are machined to the bottoms of the grooves and finished by polishing. The cited method is effective in strictly equalizing the thicknesses of the first and second birefringent regions so joined. The same applies to the joining of the third and fourth birefringent regions.
Adoption of a birefringent element thus made renders it possible to fabricate an optical isolator, for example, a polarization-independent optical isolator as a composite optical element comprising a Faraday rotator with a Faraday rotation angle of 45xc2x0 (although it is common to add an external magnetic field, spontaneous magnetization sometimes takes place in the absence of a magnetic field), at least a first and a second birefringent regions joined to one side of the rotator, and at least a third and a fourth birefringent regions joined to the opposite side of the rotator, wherein:
the light that has been transmitted through the first birefringent region passes through the third birefringent region;
the light that has been transmitted through the second birefringent region passes through the fourth birefringent region;
the optical axis of the first birefringent region and that of the second birefringent region intersect orthogonally;
the optical axis of the third birefringent region and that of the fourth birefringent region intersect orthogonally;
the optical axis of the first birefringent region and that of the third birefringent region make an angle of about 45xc2x0 with respect to each other;
the both principal planes of the first and second birefringent regions have the same surfaces flush with each other;
the both principal planes of the third and fourth birefringent regions have the same surfaces flush with each other; and
the first, second, third, and fourth birefringent regions are of the same material quality and have the same thickness d. Thus all light beams in the forward direction pass through the composite element without dependence upon the direction of polarization, whereas return light beams are all diffracted without dependence upon the polarization direction and are unable to return to the incident side.
The principles of the optical isolator described in the cited literature are as follows.
Out of the light beams incoming in the forward direction, the light of linear polarization parallel to the optical axis of the first birefringent region is transmitted as extraordinary light (refractive index ne) through the first birefringent region. As it further passes through the Faraday rotator the plane of polarization rotates 45xc2x0, and then the light passes as ordinary light (refractive index no) through the third birefringent region. [The optical path length is (ne+no)d]. Meanwhile light as ordinary light (refractive index no) is transmitted through the second birefringent region. As it further passes through the Faraday rotator the plane of polarization rotates 45xc2x0, and then the light passes as extraordinary light through the fourth birefringent region. [The optical path length is (no+ne)d].
Of the light incoming in the forward direction, light of linear polarization perpendicular to the optical axis of the first birefringent region passes as ordinary light through the first birefringent region. As it further passes through the Faraday rotator the plane of polarization rotates 45xc2x0, and then the light passes as extraordinary light through the third birefringent region. [The optical path length is (no+ne)d]. It then travels as extraordinary light through the second birefringent region and, after 45xc2x0 rotation of the polarized surface by the Faraday rotator, it passes as ordinary light through the fourth birefringent region. [The optical path length is (no+ne)d]. Thus all the optical paths are equal in length and light travels straightly forward without diffraction.
Of the light incident from the reverse direction, the light of linear polarization parallel to the optical axis of the third birefringent region is transmitted as extraordinary light through the third birefringent region. As it further passes through the Faraday rotator the plane of polarization rotates 45xc2x0, and then the light passes as ordinary light through the first birefringent region. [The optical path length is (ne+ne)d]. Meanwhile light as ordinary light is transmitted through the fourth birefringent region. As it further passes through the Faraday rotator the plane of polarization rotates 45xc2x0, and then the light passes as extraordinary light through the second birefringent region. [The optical path length is (no+no)d]. Here if d is set so that the optical path difference is 2(noxe2x88x92ne)d=(M+xc2xd)xcex (where xcex is the wavelength of the light and M is an arbitrary integer), light will all be diffracted.
Of the light incident from the reverse direction, the light of linear polarization perpendicular to the optical axis of the third birefringent region is transmitted as ordinary light through the third birefringent region. As it further passes through the Faraday rotator the plane of polarization rotates 45xc2x0, and then the light passes as ordinary light through the first birefringent region. [The optical path length is (no+no)d]. Meanwhile light as extraordinary light is transmitted through the fourth birefringent region. As it further passes through the Faraday rotator the plane of polarization rotates 450, and then the light passes as extraordinary light through the second birefringent region. [The optical path length is (ne+ne)d]. Here because the optical path difference is 2(noxe2x88x92ne)d=(M+xc2xd)xcex (where xcex is the wavelength of the light and M is an arbitrary integer), light will all be diffracted.
According to the method of Patent Application Kokai No. 11-2725, as illustrated in FIG. 1, a composite optical element is fabricated by disposing a first birefringent region 1 and a second birefringent region 2 on one side of a Faraday rotator 7 and disposing a third birefringent region 3 and a fourth birefringent region 4 on the other side of the rotator and joining them through layers of adhesive 8. The arrangement necessarily forms a fifth region 5 and a sixth region 6 of the adhesive used, in addition to the two birefringent regions each on both sides of the Faraday rotator, resulting in deteriorated optical properties. The problem arises from the fact that a blank sheet of birefringent material must be formed with a sufficient number of grooves to make a plurality of composite optical elements and, in order to bring the two grooved sheets of birefringent material face to face and into mesh with each other, adequate clearances must be provided between the grooves and lands in mesh, resulting in the formation of relatively large fifth and sixth regions.
The present invention is aimed at solving the above problem and providing composite optical elements with stabilized optical properties and also providing processes for producing the same.
The problem of the prior art can be solved by the following means according to the present invention.
(1) The invention provides a composite optical element comprising a Faraday rotator, a first birefringent region, a second birefringent region, and a fifth region, all said three regions being joined to one plane of said rotator, and a third birefringent region, a fourth birefringent region, and a six region, all said three regions being joined to the opposite plane of said rotator,
the light that has been transmitted through the first birefringent region passing through the third birefringent region,
the light that has been transmitted through the second birefringent region passing through the fourth birefringent region,
the optical axis of the first birefringent region and that of the second birefringent region intersecting orthogonally,
the optical axis of the third birefringent region and that of the fourth birefringent region intersecting orthogonally,
the principal planes of both the first and second birefringent regions having the same ground surfaces,
the principal planes of both the third and fourth birefringent regions having the same ground surfaces,
the first, second, third, and fourth birefringent regions being of the same material, and
the percentage of travel of the light beam through the fifth and six regions being no more than 10%.
(2) The invention preferably provides the above composite optical element wherein the refractive indexes of the fifth and sixth regions have values between the refractive indexes of the birefringent regions with respect to ordinary light and extraordinary light.
(3) The invention also provides a process for producing the composite optical element (1) which comprises joining a plurality of a first birefringent plates and a plurality of a second birefringent plates alternately with adhesive to form a laminate, meanwhile joining numbers of third and fourth birefringent plates alternately with adhesive to form a laminate, cutting the laminates into pieces each of which is the composite optical element, the joints formed using the adhesive constituting a fifth and a sixth regions.
(4) The invention desirably provides the process (3), wherein the refractive index of the adhesive used in alternately joining the birefringent plates into a laminate has a value between the refractive indexes of the birefringent material with respect to ordinary light and extraordinary light.
(5) The invention desirably provides the process (3), wherein only two different birefringent plates are used to form the laminate of the first and second birefringent plates and the laminate of the third and fourth birefringent plates.
(6) The invention desirably provides the process (3), wherein the positional relations of the four different birefringent regions can be decided without adjustment, through control of the thickness of the birefringent plates to be the same.
(7) The invention further provides a composite optical element comprising a Faraday rotator, a first birefringent region and a second birefringent region joined to one plane of said rotator, and a third birefringent region and a fourth birefringent region joined to the opposite plane of said rotator,
the light that has been transmitted through the first birefringent region passing through the third birefringent region,
the light that has been transmitted through the second birefringent region passing through the fourth birefringent region,
the optical axis of the first birefringent region and that of the second birefringent region intersecting orthogonally,
the optical axis of the third birefringent region and that of the fourth birefringent region intersecting orthogonally,
the principal planes of both the first and second birefringent regions having the same ground surfaces,
the principal planes of both the third and fourth birefringent regions having the same ground surfaces,
the first, second, third, and fourth birefringent regions being of the same material, and
the first and second birefringent regions and the third and fourth birefringent regions are each joined by optical contact.
(8) The invention also provides an optical isolator comprising a composite optical element (1), (2), or (7), which further comprises at least one lens and one optical fiber arranged on each of the two sides of the composite optical element. A magnetic field is applied by a magnet to the Faraday rotator of the composite optical element, but the magnet is not needed when a material that spontaneously magnetizes itself is used in the Faraday rotator.
(9) The invention further provides an optical isolator comprising the composite optical element of (1), (2), or (7), and at least one optical fiber arranged on each of the two sides of the composite optical element. As for the means of magnetizing the Faraday rotator, the same as (8) above applies.
(10) The invention preferably provides the isolator (9), wherein the optical fibers are arranged in a capillary and the composite optical element is fitted in a groove formed in the capillary.
(11) The invention further provides an optical attenuator comprising at least a composite optical element and magnetic field-applying means, said optical element comprising a Faraday rotator, a first birefringent region and a second birefringent region joined to one plane of said rotator, and a third birefringent region and a fourth birefringent region joined to the opposite plane of said rotator,
the light that has been transmitted through the first birefringent region passing through the third birefringent region,
the light that has been transmitted through the second birefringent region passing through the fourth birefringent region,
the optical axis of the first birefringent region and that of the second birefringent region intersecting orthogonally,
the optical axis of the third birefringent region and that of the fourth birefringent region intersecting orthogonally,
the principal planes of both the first and second birefringent regions having the same ground surfaces,
the principal planes of both the third and fourth birefringent regions having the same ground surfaces,
the first, second, third, and fourth birefringent regions being of the same material, and
said magnetic field-applying means being one capable of applying a variable magnetic field to said Faraday rotator.
(12) The invention provides the attenuator (11), which further comprises at least one lens and one optical fiber arranged on each of the two sides of said composite optical element.
(13) The invention also provides the attenuator (11), which further comprises at least one optical fiber arranged on each of the two sides of said composite optical element.
(14) The invention preferably provides the attenuator (11), wherein the optical fibers are arranged in a capillary and the composite optical element is fitted in a groove formed in the capillary.
(15) The invention provides the isolator (8), (9), or (10), wherein said composite optical element is located as inclined to the optical axis of incident light.
(16) The invention provides the attenuator (11), (12), (13), or (14), wherein said composite optical element is located as inclined to the optical axis of incident light.
(17) The invention further provides the isolator (15), wherein said composite optical element is located as inclined to the optical axis of incident light, and
the plane including said optical axis and normal of the plane of incidence of said composite optical element is parallel to the interfacial boundary between said first and second birefringent regions.
(18) Thus the invention further provides the isolator (17), wherein
the interfacial boundary between said first and second birefringent regions and
the interfacial boundary between said third and fourth birefringent regions are on the same plane.
(19) The invention also provides the attenuator (16), wherein
said composite optical element is located as inclined to the optical axis of incident light, and
the plane including said optical axis and normal of the plane of incidence of said composite optical element is parallel to the interfacial boundary between said first and second birefringent regions.
(20) The invention also provides the attenuator (19), wherein
the interfacial boundary between said first and second birefringent regions and
the interfacial boundary between said third and fourth birefringent regions are on the same plane.
The attenuators (19), (20) are directly applicable to the embodiment of optical isolator described with reference to FIG. 15, in which case, however, a variable magnetic field-applying means becomes necessary.
(21) The invention also provides an optical attenuator comprising the composite optical element (1), (2), or (7), magnetic field-applying means, and at least one lens and one optical fiber arranged on each of the two sides of said composite optical element.
(22) The invention also provides an optical attenuator comprising the composite optical element (1), (2), or (7), magnetic field-applying means, and at least one optical fiber arranged on each of the two sides of said composite optical element.
(23) The invention further provides the (22), wherein the optical fibers are arranged in a capillary and the composite optical element is fitted in a groove formed in the capillary.
With regard to (21), (22), and (23) above, the constructions described in connection with optical isolators can be utilized as they are by simply modifying the variable magnetic field-applying means.